When animal cells divide they first round into a ball, that cleaves in the middle to form two daughter cells, then the two daughters spread out again. Understanding how these shape changes are controlled will provide basic knowledge relevant to human development and tissue formation, and might provide clues to new methods for blocking cell division in cancer cells. Cell shape is governed by the cytoskeleton, a system of fibers in the cytoplasm made of the subunit proteins actin and tubulin. Cell probably change shape during division controlling addition and loss of subunits from these fibers. We will use biochemistry and microscopy o understand the addition and loss reactions. The food-poisoning bacterium, enters human cells and generates a "comet tail" of actin fibers that propels the bacterium. By studying actin fiber formation and loss n these tails we will learn how subunit addition and loss is normally controlled during cell division, n aim 1 we will analyze how actin subunits are lost from polymers, both in single filaments and in Listeria tails. For example, are subunits lost from the ends of actin filaments, or does the whole filament break apart as it disassembles? We have purified three proteins that catalyze subunit loss, and we will determine how they work. In aim 2 we will ask how nucleation of new actin filaments in the cytoplasm is controlled during cell division, using extracts from frog eggs. We hypothesize that nucleation is globally stimulated in mitosis, and inhibited during cleavage. We will test this idea, and determine its biochemical basis. In aim 3 we will ask how an asymmetric assembly of microtubules call the midbody is formed as cells cleave, and how the polymers in this architecture are stabilized. Understanding midbody assembles will reveal principles for generating asymmetric assemblies in the cytoplasm, and will tell us how the cleavage furrow is positioned.